1. Field of the Invention
The present invention relates to an automatic transmission having a planetary gear unit, in detail to a supporting structure of a ring gear.
2. Description of the Related Art
The applicant of the present invention has proposed, as shown in Japanese Laid Open Patent No. Sho-62-141341, an automatic transmission including an automatic transmission mechanism attaining forward three speed by the following structure: the automatic transmission mechanism attaining forward three speed includes a planetary gear having a single and dual planetary gears; sun gears of both planetary gears are commonly linked; carriers of both planetary gears are commonly linked; the carrier is connected to a counter (drive) gear which is an output member; an input shaft and a ring gear of the single planetary gear are connected through a first clutch; the input shaft and the sun gear are connected through a second clutch; the sun gear is restrained by a first brake or a second brake through a first one-way clutch; a ring gear of the dual planetary gear is restrained by a third brake or a second one-way clutch; the forward three speeds are obtained by operation of the respective clutches and brakes at required moment.
A front end of the ring gear of the dual planetary gear, or a big diameter ring gear is fixed with an outer race of the second one-way clutch, while a rear end of the big ring gear supports and fixes a supporting plate extending inner side. The supporting plate is rotationally supported between a front side carrier and the ring gear of the single planetary gear through thrust bearings.
Because of the above structure, the thrust bearing which supports the big ring gear has to be situated outside from the ring gear of the single planetary gear, or a small diameter ring gear, so that a portion where thrust bearings are located has bigger radius, and subsequently, rotative speed there is higher. Accordingly, problems on durability of the bearings may occur.
Assembly work is quite laborious because one end of the ring gear of the dual planetary gear is fixed on the outer race of the one-way clutch, and another end is fixed on the supporting plate which is situated radially downward.